Can a Perlite and Portland cement mix handle high temperature?

Hi everyone!

I’m hoping to build my first bread/pizza oven using perlite. I’m kinda confused about Portland cement. I’ve read in the book “The Bread Builders” that Portland cement starts to lose strength around 450°, which suggests to me it won’t handle higher temperatures.

So when I’m making the inner wall of the oven (closest to the flames) what should I mix with the perlite that can withstand high heat? The same book also says clay isn’t great either. Home Depot sells refractory cement in 25lb tubs. Would that work?

Hi Bradley, and welcome to the BrickWood forums.

If you are looking at the instructions for a BrickWood oven, I think you are confusing the directions for the insulating base with those for the high temperature mortar.

The thing they have in common is that they both use Portland cement as a binding agent. But they are wildly different in purpose.

For the insulating base, you are using Perlite mixed with Portland cement as a thermal break to keep your hearth slab from drawing away heat from the oven door. It never sees direct heat, not does it come in contact with the firebrick in the hearth.

For the oven frame and arch, you need a high temperature mortar. Portland cement is one ingredient in that blend as well, but you also have silica sand, hydrated lime, and most importantly fireclay in the mix. There is NO Vermiculite or Perlite in the mortar. The combined ingredients give the mortar mix high temperature resistance and structural durability that would never be achieved by Portland cement on its own.

You can also use a DRY high temperature mortar such as HarbisonWalker KS4 (the route I chose), but it will add significantly to the finished cost of your build.

I highly recommend the simple mix for the insulated base, and it’s your call (and wallet) as to whether you want to mix your own high temp mortar or buy it as a dry mix. You MUST NOT use a wet premixed product in a pail; it is not suitable for this use.

Please keep posting questions after you’ve looked over the oven plans again. There are plenty of areas for creativity here, but your refractory materials need to hew to specifications if you want the oven to perform.

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Can you use regular mortar under the the slabs and in between them

Good question, and the answer is yes. No heat there, and I recommend you mix up a good-sized batch and lay a bed of it on top of the blocks for each of the three slabs.

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Thank you I thought it would be ok

Hi Matt
Instead of using the standard brick for the frame I’m using 2” thick Blustone do I need to use high heat mortar go set them or just regular mortar.

Dale Leger

I’d use the high temp. You want to avoid eventual breakdown due to heat, which standard mortar will show ( over a period of years). See our discussion above; the difference here is your framing brick or stone is up on the “mezzanine level.” It will be exposed to heat in a way the slabs will not.

Thanks Matt,
I made the slabs with 2X8’s to give me extra height and 2 pieces of 2” styrofoam to give me 4” in the pour witch will give me 3 1/2 “ of concrete 11/2” of perilite cement mix 1”” of sand then the fire brick cooking surface to make even with the bluestone.
Had to set the slabs with my tractor , they were a little heavy. Added a lot of rebar also.

Thanks Again

Does the sand under the cook surface need to be compacted


Hi Dale,

Yes, it does.

I wrote a guide about it.

You want a very stable and level surface to support your hearth firebrick.

Hope this helps!

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I am using the vermiculite/portland mix as the insulating base. My question is: How soon after laying this base can I proceed with adding the sand and installing the firebrick oven floor?

Hi Pete, and welcome to the BrickWood forum!

You need to allow the Portland cement some time to set up. With Vermiculite, it will take about 5 to 6 days per the base installation instructions. You can shave 4 days off that time by using Perlite instead of Vermiculite. They are both volcanic-based materials, and are about equivalent in insulating value, but Vermiculite retains moisture while Perlite sheds it. They also cost about the same.

Good luck and let us know how it goes!

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